Microfilm is commonly used to archive documents and substantially reduces the volume required for storage of documents due to a significant reduction in document (image) size. To read or view a document which has been placed on microfilm a microfilm reader, printer or reader/printer is utilized. The microfilm reader and/or printer enlarges the image (document) to viewable or printable size.
Micrographic readers, printers or reader/printers typically require variable magnification ratios and typically must use up to 35 millimeter format microfilm. Microfilm currently existing in the market is filmed at a variety of reductions. Hence, a single magnification will not provide optimum enlargement for more than a single reduction.
When a user possesses microfilm recorded at different reductions, it is necessary to have a means to vary the enlargement to bring either the viewed or printed image back to the original dimensions. Moreover, it is sometimes desirable to bring back the image at a different magnification, either greater or smaller, than the magnification at which the original image was filmed.
There are three basic techniques to optically vary the magnification.
The first technique is to use a separate lens for each magnification. Since a separate lens must be used for each magnification, this "traditional" approach results in users requiring a multiplicity of lenses. This leads to increased cost, complexity and inconvenience of switching lenses. Further, this technique has the disadvantage of providing only discrete steps for magnification. For example, if a reader and/or printer provides 10x, 20x and 30x magnifications, three separate lenses are required. If a 24x magnification is desired, an additional separate 24x lens must be provided. If finer increments, say 0.5x, are desired, the number of additional lenses becomes, at best, impractical and, realistically, cost prohibitive.
The second technique is to vary the object to image distance. This technique is commonly used in photocopying machines and has been incorporated into some microfilm machines. This technique allows a single lens to provide multiple magnifications by varying the distance from the object (film) plane to the print plane. Such fixed focal length lenses do not provide optimum correction at multiple object to image distances and must still be refocused after each change in path length. Moreover, if a substantial range is to be covered, the path length must vary considerably. For example, if a 90 millimeter focal length lens is used to cover a magnification range of 8x to 30x, the path length will need to change by nearly 2 meters. Multiple paths or multiple lenses will alleviate this problem somewhat, but at the expense of added cost and complexity.
The third technique is to utilize a zoom lens which allows a stepless variation of magnification with a single fixed optical path.
Zoom lenses have been used in micrographic applications since at least the mid-nineteen eighties. Some of these zoom lenses are disclosed in U.S. Pat. No. 4,733,951, U.S. Pat. No. 4,743,102, U.S. Pat. No. 4,746,204 and U.S. Pat. No. 4,750,820, providing magnification ratios of 6.5x-14x, 12x-24x, 14x-32x and 20x -47x, respectively. These lenses are typically of two or three group construction. They are intended primarily for use with 16 millimeter microfilm and/or microfiche images and do not cover large format, i.e., 35 millimeter, microfilm images. Moreover, none of these lenses allows magnification to vary by more than a factor of 2.35 to 1.
A four group zoom lens is used in the 3M "Q-IV" printer, available from Minnesota Mining and Manufacturing Company, St. Paul, Minn., the assignee of the present invention. This zoom lens provides a 7.5x-24x magnification variation, a factor of 3.2 to 1, but is not capable of providing full-frame viewing on an 18 inch (45.7 centimeter) by 24 inch (61.0 centimeter) screen and is not capable of printing "E" size, i.e., 18 inch (45.7 centimeter) by 24 inch (61.0 centimeter), documents.